A particular genetic variant makes people much more susceptible to post-traumatic stress disorder (PTSD) after harrowing experiences, but only if they have also had an abusive childhood, US scientists have discovered. The findings add to a growing consensus that the debate about whether mental health, personality and behaviour are driven by nature or nurture is founded on a misconception. They indicate strongly that genes and the environment are not mutually exclusive forces, but rather work together to influence human development.

PTSD is a serious anxiety disorder that develops among people who experience unpleasant events, such as war, murders, terrorist attacks or natural disasters. It leads to nightmares, insomnia, flashbacks, mood swings and depression, and can severely impair the ability to live a normal life.

Not everybody who experiences severe trauma develops PTSD, and the risk is known to be influenced by genetics. Studies of twins who served in Vietnam showed that identical pairs, who share all their genes, are more likely both to suffer than are fraternal sets.

Genes, however, do not explain all the variability in peopleâ€™s risk, and the precise genes and environmental factors that are involved have remained obscure.

A study led by Kerry Ressler, of Emory University in Atlanta, examined the effects of a gene called FKBP5, which is involved in the way the body responds to stress. The DNA code of this gene varies at four points, which allowed the scientists to investigate whether any particular genetic profiles would either raise the risk of PTSD or protect against it.

As PTSD develops only when people have lived through traumatic events, Dr Ressler studied a group of 900 adults who lived in deprived urban communities and were likely to have had violent experiences of the sort that can provoke the disorder.

The participants were also asked to complete a questionnaire that recorded whether they had suffered physical or sexual abuse at a young age. When variations in the FKBP5 gene were examined on their own, the researchers found no effect on PTSD risk. A history of child abuse also made no difference in isolation.

When the two factors were considered together, however, they were found to interact to raise or reduce risk. People with certain variants of FKBP5 were much more likely to develop PTSD after trauma if they had also been abused as children.

â€œThese results are early and will need to be replicated, but they support the hypothesis that combinations of genes and environmental factors affect the risk for stress-related disorders like PTSD,â€ Dr Ressler said.

â€œUnderstanding how gene-environment interactions affect mental health can help us to understand the neuro- biology of these illnesses.â€

A team led by Avshalom Caspi and Terrie Moffitt, of the Institute of Psychiatry, London, has found that a variant of a gene called MAOA predisposes to antisocial behaviour when accompanied by child abuse. Dr Caspi said: â€œIt is part of an emerging body of research that documents not so much that genes cause disease, but rather that genetic differences shape how people respond differently to the same events.â€